Chapter 1

Introduction

Most of us take for granted our ability to perceive the external world. However, this is no simple deed at all.Imagine being given a task of designing a machine that can perceive, locate, describe and identify all the objects inthe environment and their relationship with each other. Such a machine is yet to be realized. However, humans andanimals have solved this problem in an seemingly effortless manner. Because of this ease, with which we perceivethe environment around us, often we do not realize the complexity and the importance of the feat achieved.

1.1 The Perceptual Process

Figure 1.1: The Perceptual Process

First, let us introduce the perceptual process. This process is diagrammed in Figure 1.1 as a sequence of stepsstarting from the environment to the perception of a particular stimuli, recognition of the stimuli and action withregard to the stimuli. We will do this with an example. Let us take an example of Mary who has just arrived at thecircus.

The environmental stimulus is defined as all the things in our environment that we canpotentiallyperceive. Inour example, as Mary takes her seat, she observes everything around her: an animal act going on in the centerstage, a few clowns in the side, the audience around her, the trapezes hanging from the ceilings and the music bandon the right. These form the environmental stimulus.

1

2 CHAPTER 1. INTRODUCTION

Next, Mary focuses her attention to something that she finds particularly interesting, for example the animal actgoing on in the center stage. This is calledattended stimulus.

When Mary focuses her attention on the animal by looking directly at it, a image of the animal is formed on thereceptors of the retina of the eye. This defines thestimulus on the receptorsstage.

Transductionis the process of transformation of one form of energy to another. For example, when you pressany button on the bank teller machine, the pressure produced by your finger is transducted to electrical energy.Similarly, in the nervous system, when environmental energy is transformed to electrical energy, it is called trans-duction. In our example, this occurs when the stimulus on the receptors are transformed into electrical signals.

These electrical signals generate new signals in the cells of nervous system called neurones. This process iscalledneural processing.

Perceptioncan be defined as a conscious sensory experience. When the electrical signals produced by theneurones are somehow processed by the brain and converted to the experience of seeing the animal, the stimulusis said to be perceived.

The next two processes, recognition and action are not exactly perception. They are perceptual behaviorsthat are important outcome of perceptual process.Recognitionis our ability to place a object in a category. Forexample, Mary is at this point identify the animal to be a tiger. Though it may be tempting to believe that perceptionand recognition happen together, it has been shown that they are different.Action includes the motor activitiessuch as moving the eyes or moving the body in response to the process of perception. Almost always an actioninvolves movement of the eye, which changes the attended stimulus and the whole cycle repeats.

Knowledgehas a role to play in the ability of an individual to recognize different objects, events or situations.For example, using previously acquired and stored knowledge of different animals, Mary is able to identify theanimal in the center stage as a tiger. Thus, knowledge is involved while perception and triggers recognition.

The inclusion of knowledge in this cycle enables us to distinguish between two types of perceptual processing.

• When the process starts from the stimulus on receptors and goes towards perception, it is bottom up pro-cessing.

• The process that starts with the knowledge that a person brings to an environment is a top down processing.

We will see in the rest of the course, that perception often involves both of these working together.

1.2 Visual Perception

There are several sense which help us to perceive the world around us. These include seeing, hearing, touching,tasting and smelling. There are some creatures for whom nonvisual senses play the dominant role. For example,bats use there audio perception for navigation. However, for humans and for most other species, vision plays themost important role. Vision has evolved in these animals to provided successful means of survival and reproduc-tion. It is through vision that one can achieve desirable objects like nutritious food, warm shelter and strong mates.It also provides important cues against dangers like predators and falling objects.

The primary advantage of vision, which is probably the reason for its evolution as the preeminent vehicle ofperception, is that it provides accurate information from a distance. The sense of hearing and smelling can provideinformation from a distance, but may not be very accurate. At the same, tasting and touching can provide accurateinformation, but only on contact. Thus, vision provides, what we callveridical perception(in Latin veridicusmeanstruthfully) consistent and accurate with the actual state of the environment without being in contact withthe environment. Though there are a small number of pathological cases when this is not true, but for amazinglylarge number of situations, by and large,what we see is what we get.

In the context of this course,visual perceptionis defined as the process of acquiring knowledge about ourenvironmental objects and events by extracting information from the light they emit or reflect. Note that visual

1.3. STUDYING THE PERCEPTUAL PROCESS 3

perception involves acquisition of knowledge. This means that it is a cognitive activity and is not merely anoptical process. This is the different between human vision and cameras. Cameras can take pictures but do notknowanything about the scenes/objects they capture. Cameras can merely captureinformation, but humans andanimals can additionallyacquire knowledgefrom this captured information.

1.3 Studying the Perceptual Process

There are primarily two different ways to study the perceptual process. In the first, we study how the person’sperception is related to the stimulus. Here we do not consider the details ofhowthis perception was really arrivedat. This is calledpsychophysical analysisof the perceptual process. The second way to study it is to considerhow the person’s perception is related to the physiological processes that are occurring within the person’s sensorsand/or brain. This is calledphysiological analysisof the perceptual process. Both of these are indicated in Figure1.1. In both these, we will also be concerned with the role of knowledge (includes also memories, expectationsetc) in perception. This is calledcognitive influences. However, note that all these are intimately related and oftenwe will take a more holistic approach of cross referencing these disciplines and forming a complete and consistentpicture of the perceptual procedure.

1.3.1 The Psychophysical Approach

The relationship between stimulus and perception can be measured in three different ways. There are two waysof measuring it qualitatively. These aredescriptionandrecognition. In the descriptive way, the subject is askedto describe a perception. In recognition, the subject is presented with a stimulus and asked to recognize. Both ofthese are heavily used by doctors in treating patients. However, with the goal of studying perception, what we aremore concerned with is a quantitative way of measuring the relationship, throughdetection,magnitudeandsearch.

The method of detection was first proposed by the famous scientist Gustav Fechnar in 1860 and was later im-proved by scientists like Ernst Weber.

Absolute Threshold: This is defined as of the smallest amount of stimulus required to detect the stimulus. Fechnerproposed three different methods to find the absolute threshold.

• Method of Limits:Here the subject is presented with stimulus in an ascending or descending order and thesubject is asked to detect at which point the stimulus becomes detectable or undetectable respectively.

• Method of Adjustment:Here the subject is provided with a continuous control by which he/she can controlthe strength of the stimulus and then mark the point where it is barely detectable.

• Method of Constant Stimuli: Here the subject is presented with stimuli of random strength at random orderand asked to identify if it is detectable or not. The date thus collected can be plotted to identify the range ofthe absolute threshold.

Difference Threshold: This was proposed by Ernst Weber. The difference threshold (DL) is defined as thesmallest difference between two stimuli that a person can detect. To measure the difference threshold, a personis presented with a ‘standard’ stimulusS and a ‘compared’ stimulusC simultaneously. The difference betweenthe strength of the ‘compared’ stimulus and the ‘standard’ stimulus is increase by increasing the strength of the‘compared’ stimulus until the difference is detected by the subject.

Studying the difference threshold for various stimulus led to the discovery of a very important relationship inperception called theWeber’s Law. This says that the ratio of the difference threshold to the standard stimulus isconstant. Mathematically,

DL

S= K, (1.1)

4 CHAPTER 1. INTRODUCTION

Figure 1.2: Left: Response Compression and Expansion, Right: The same response is linear in a logarithmic scale.

where the constantK is called theWeber fraction.These methods of measuring thresholds tell us when a stimulus becomes detectable. But what about the above

threshold perception. For example, if we double the intensity of a light, does it look doubly bright? The methodof magnitude is used to study this.Magnitude Estimation: To measure the above threshold relationship between stimulus and perception accurately,S. S. Stevens developed a method calledmagnitude estimation. In this method, the subject is presented withstimulus of different strengths, and is asked to assign a number to his perceived sensation. This numbers areplotted against the strength of the stimulus presented to get the response.

It has been found that for most sensations, doubling the stimulus does not necessarily mean doubling in theperceived sensations. Thus, the relationship between stimulus and perception is not linear. For some sensationslike brightness of light, the change in the perceived sensation is less than the change in the stimulus strength.This is calledresponse compression. For some other sensations like electric shock, the change in the perceivedsensation is more than the change in the strength of the stimulus. This is calledresponse expansion. These areillustrated in Figure 1.2. In fact, Steven discovered that for most senses, the strength of the stimulus,S, and theperceived sensation,P , is related by a power function,

P = KSn, (1.2)

whereK andn are constants.n > 1.0 suggests response expansion andn < 1.0 suggests response compression.This is calledSteven’s Power Law.

Finally, the method of search involves looking for and recognizing. This can often present important insightsfor the relationship between stimulus and perception.

The above gives a few extensively used methods used for psychophysical experiments. But, several othermethods are used or are being devised to find the relationship between stimulus and perception.

1.3.2 The Physiological Approach

Receptors in the sensory organs like eye, ear, skin etc transduce environmental energy (stimulus) into electricalsignals. These signals are carried to different areas of the brain bynerveswhere they are processed to cause

1.3. STUDYING THE PERCEPTUAL PROCESS 5

Figure 1.3: A nerve is made up of many nerve fibres.

different perceptions. This is the broad basis of the physiological approach of studying the perceptual process.Nervesare composed of smaller structures calledneurons. Neurons consist of

1. Cell Body:This contains the nucleus and other metabolic structures required to keep the cell alive.

2. Dendrites:This branch out from the cell body to receive electrical signals from other neurons.

3. Axon or Nerve Fibre:A tube filled with fluids that conducts the electrical signal.

Many such neurons combine to create a nerve. The axons form the individual components of the nerve, just likemany individual wires travel within a telephone cable as shown in Figure 1.3.Receptorsare specialized neuronsthat can respond to environmental stimuli like light or sound waves.

Recording of Electrical Signals in Neurons:Neurons carry electrical signal in a very different way that the wiresthat carry electricity to our homes. Neurons are immersed in a solution rich inions, molecules that carry electricalcharge. The solution outside the axon of a neuron is rich in positively charged sodium (Na+) and the solutioninside the axon is rich in positively charged potassium (K+). These ions create electrical signals in the neuronwhen they flow across the axon cell membrane.

Figure 1.4 show how this happens. The example shows a pressure sensitive receptor neuron. In the normalstate (not excited), a potential difference of70 mV exists across the axon cell membrane. This is caused by thedifferent concentration of the ions inside and outside the axon. This is called theresting potential.

When excited by the pressure stimulus, a rapid change in the potential occurs. It goes up rapidly to40 mV andreturns back to the resting potential, all within the very small time of1 ms. This rapid increase in the potential iscalled a nerve impulse or anaction potential. This caused by the flow of the ions across the cell membrane. Therising phase of the action potential is accompanied by a inflow ofNa+ in the axon raising its positive potential.This is followed by an outflow ofK+ from the axon which brings down the action potential back to the restingpotential. This kind of flow of ions are caused by the change inpermeabilityof the membrane to sodium andpotassium. Permeability is a property of the membrane that refers to the ease with which molecules can passthrough the membrane. When the cell is not stimulated, the permeability to sodium and potassium is low. Thestimulation triggers a process by which the membrane becomes highly permeable to sodium and potassium leadingto the action potential. However, theNa+ andK+ does not accumulate inside and outside the axon respectively.This is prevented by a mechanism calledsodium-potassium pumpwhich continuously returns sodium to the outside

6 CHAPTER 1. INTRODUCTION

Figure 1.4: Generation of nerve impulse. As long as the fibre is at rest, there is a resting potential of−70 mVbetween the inside and outside of the fibre (a). As a nerve impulse is generated, an increase in potential occurs byinflow of Na+ in the axon (b). This is followed by a outflow ofK+ from the axon that brings the potential backto−70 mV (c)(d).

and potassium to the inside of the axon at their original concentration levels so that the axon does not continue togenerate nerve impulses. Next, this action potential travels all the way down the axon. This is calledpropagatedresponse.

However, the resting potential, the action potential and the duration of the action potential is always the same.This means that the strength of the stimulus is not indicated by the magnitude of the potential. Instead, it is indi-cated by the rate of firing. Higher the stimulation, higher is the rate of firing. However, there is maximum limit tothe rate of firing. This is due to arefractory periodof 1 ms after each firing when the neuron cannot fire. This setsthe upper limit of the firing rate to about800 impulses per second. Another important point to note here is that inthe non-excitatory state also the neurons show some minimum firing. This is called thespontaneous activity. Thisplays an important role in perception as we will find later.

Transmitting the Electrical Signal: The next question is once the action potential is transmitted through the axonas propagated response, how does it get transmitted to other neurons? The neurons are not in contact with eachother. There exists a very small space between two neurons called thesynapse. The electrical signals does not go

1.3. STUDYING THE PERCEPTUAL PROCESS 7

across this synapse. Instead they trigger a chemical activity in the synapse which is instrumental in transmittingthe information from one neuron to another.

Figure 1.5: The neuron shown on the right has a cell body, dendrites and axon. A neuron that receives stimulifrom the environment, shown on left, has a receptor in place of cell body.

The neuron from which is transmitting the signal is called thepresynaptic neuronand the neuron receivingthe signal is called thepostsynaptic neuron. Thesynaptic vesiclesof each neuron store someneurotransmitters.When the action potential reaches the end of presynaptic neuron, these vesicles release packets of neurotrans-mitters. These flow into small areas of the postsynaptic neuron called thereceptor sites. Each receptor site isof different shape and sensitive to specific neurotransmitters. When the neurotransmitter makes contact with areceptor site that matches its shape, the postsynaptic neuron is triggered. However, depending on the nature ofneurotransmitter and the nature of cell body, it can cause either anexcitation(increased firing) orinhibition (re-duced firing). The necessity of inhibition will become evident as we advance through the course. This wholeprocess of generation and movement of electrical signal through the neurons are illustrated in Figure 1.5

Processing in Brain: Finally, the signal thus transmitted reaches the brain. The brain is still an active area ofresearch and here we will cover whatever little we know about it. Much of the research in the connection ofperception is focussed in the area ofcerebral cortex. It is a 2 mm this layer that covers the surface of the brainand contains the machinery for creating perception, language, memory and thinking. A basic principle of corticalfunction ismodular organization– specific functions are served by specific areas of the cortex.

There are differentprimary receiving areasfor different senses in the brain. Each of these areas receive signalsinitiated by that particular sense’s receptors. For example, theoccipital lobe is the primary receiving area forvision, temporal lobefor hearing andparietal areafor touch. However, advanced processing of the signals in thebrain are not limited to the primary receiving areas.